The invention relates to the field of thin film photovoltaic (PV) technology. PV devices use sunlight to generate clean, reliable electric power without the environmental problems of fossil fuels or nuclear power. For such PV cells to become a serious and economically attractive alternative energy, they need to be provided in a suitable form in large quantities and made by relatively low-cost processes, using fairly inexpensive raw materials.
The copper indium diselenide (CIS) thin-film PV cell combines excellent efficiencies and long-term stability with low cost potential. The state-of-the-art CIS PV technology uses a p-type copper indium gallium diselenide (p-CIGS) absorber layer. The p-CIGS device typically uses Mo/p-CIGS/n-CdS/ZnO/metallization layers sandwiched between two rigid, fragile and heavy glass panels. This device could be also produced in a flexible configuration, whereby a single metal foil or plastic substrate replaces the two glass panels of conventional rigid modules.
Flexible, lightweight solar cells offer distinct advantages over the latter. They are amenable to monolithic roll-to-roll processing on continuous metal foils. These cells require no grid for current collection, they can be ultra thin (<100 μm), lighter and less fragile than rigid glass modules. They may be bent for manufacturing flexible solar panels. Since they mitigate much of the balance-of-system costs (frames, etc.), they have potentially low material and production costs. These aspects of flexible cells that reduce the weight, fragility and balance-of-system costs allow them to work in many non-utility applications, such as electric vehicles, building-integration and mobile systems. They can provide higher specific power ratings for space applications.
U.S. Pat. No. 5,356,656, No. 6,509,204 and the included references describe different aspects of fabricating amorphous Si solar cells. They describe a roll-to-roll method for depositing solar cell on a polymer film substrate. Amorphous Si solar cells are now routinely produced on flexible metal or metal-coated polymer substrates. U.S. Pat. No. 6,310,281, No. 6,372,538, No 6,429,369 and No. 6,441,301 adapt the flexible PV cell structure and the roll-to-roll method for fabrication of CIGS thin-film modules. The roll-to-roll type process offers a continuous-motion, high volume production. However, the current methods available for CIGS deposition use mainly vapor phase methods, which are complex, hazardous and extremely expensive. They require multi-source nozzles, chambers and vapor-creating systems. They use very high temperature, which precludes deposition of CIGS on temperature sensitive polymer substrates.
Roll-to-roll deposition method is widely used in the electroplating and electromachining industry. It is thus amenable to mass-producing CIS cells inexpensively. U.S. Pat. No. 6,117,703 adopts the electrodeposition approach for deposition of a CuInS2 absorber based PV cell. Despite a solar-matched bandgap the CuInS2 absorber has been inherently of a lower PV efficiency relative to the CIGS absorber. The CuInS2 material has not demonstrated good PV performance even when produced by the more expensive vapor deposition techniques or in single crystal form. The presence of many secondary phases also impacts the device stability.
U.S. Pat. No. 6,429,369 proposes to use the electrochemical method for deposition of p-CIGS cell. Adapting the electrodeposition approach to the more efficient p-CIGS absorber to produce a cost effective flexible cell configuration poses many technical and economical challenges: (a) the p-CIGS device requires a sputtered Mo layer or Mo-foil contact, which is too expensive for large-scale terrestrial applications; (b) CIGS electrodeposition on Mo coated metal substrates requires a conducting diffusion barrier between Mo and the metal; (c) the co-electrodeposition of Ga, S and Na with Cu, In and Se is extremely difficult: many of these elements have to be added by vapor deposition. These additional materials and process steps complicate the fabrication process and add higher costs. The need for chemical bath deposition of a CdS buffer layer and KCN etch for the CIGS and CuInS2 based PV cells further add to the hazards and the waste disposal expense.
U.S. Pat. No. 6,228,243 B1 describes a molecular layer electrodeposition (MLE) method to create high quality semiconductor thin films using a thin layer flow cell. This approach, controlled at molecular level, entails the successive electrodeposition of monolayers of a compound from a single electrolyte to form a superlattice.
U.S. Pat. No. 4,601,960 describes the fabrication of n-copper indium selenide (n-CIS) based photoelectrochemical cell and the electrochemical formation of a p/n heterojunction. Menezes demonstrated over 12% efficiency for an n-CIS single crystal cell in a photoelectrochemical configuration. U.S. Pat. No. 5,286,306 adapts the heterojunction formation approach described in U.S. Pat. No. 4,601,960 to make a solid-state n-CIS thin-film cell. The invention of U.S. Pat. No. 5,286,306 could only be partially reduced to practice because a reliable deposition method for the n-CIS thin film and a suitable front contact were unavailable. Light-blocking metallization was used to make the front contact to the device, which impeded its energy conversion efficiency assessment. The n-CIS thin film device could eliminate many of the difficulties encountered with the p-CIGS and n-CuInS2 devices, if a viable device configuration, based on a wider bandgap absorber and a viable manufacturing method were devised for high volume production.
Accordingly, besides the objectives and advantages of my previous U.S. Pat. Nos. 5,286,306 and 6,228,243 B1, the main objectives and advantages of the present invention are related to the design of (a) a new thin-film PV solar cell configuration that:                is inexpensive, safe and simple to manufacture in high volume        eliminates the complex processing, toxic and expensive components of the CIGS solar cell, but retain its optimal PV properties        minimizes the number of device components,        
and (b) a new fabrication method that                is simpler and cheaper than the state-of-the-art        uses fewer processing steps        low temperature processing        specifically avoids hazardous materials and processing steps        can be made on flexible metal or polymer substrates in a continuous roll-to-roll process        adapts to rigid substrates, bifacial illumination and superstrate configurations.        